1. Field
The present invention relates to a transmission system having a transmission-side device transmitting data, and a reception-side device receiving the data from the transmission-side device.
2. Description of Related Art
Recently, in order to achieve efficiency improvement in networks, such as cost reduction and increase in communication speed, there is a growing tendency toward converting conventional networks into IP (Internet protocol)-based networks. Thus, the conventional TDM (time division multiplex) typified by SONET (synchronous optical network)/SDH (synchronous digital hierarchy) is being shifted to the IP. Under such an environment, there occurs the need for accommodating the conventional TDM network in a new IP network. In this case, a TDM signal is packetized on the transmission side, and after having been transmitted via the IP network, it is regenerated on the reception side. At this time, however, the regeneration of a clock signal poses a problem.
The cellification of TDM signal has been implemented by an AAL1-based circuit emulation technique, in conformity with ATM (asynchronous transfer mode). Because the ATM is constituted of a cell having a fixed and short data length, it has been little affected by delay or jitter, thereby allowing clock regeneration to be comparatively easily performed. On the other hand, the technology for implementing the IP-packetization of a TDM signal is generally referred to as “pseudo wire” or “PWE3” (pseudo wire emulation edge to edge). The IP packet has a non-fixed and variable length; in recent years, a jumbo IP packet (up to 9600 bytes) has made its market debut. In the IP packet, a variety of pieces of data are mixed, and thus QoS (quality of service) control with respect to the network is insufficient, so that packet arrival intervals on the reception side considerably vary. A typical clock regeneration method on the reception side is to regenerate a clock upon having checked the amount of received data. However, if arrival intervals of received packets vary, jitter becomes large, so that a regenerated clock may differ from the clock on the transmission side. In particular, in the event that packet is lost, the clocks on the transmission and reception sides may significantly deviate from each other.
A technique for matching sampling frequencies on the transmission side and the reception side in a transmission system, is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2006-157361.